Over the past 10 years a new generation of catalysts which utilize alumoxane as an activator for mono, bis or tris cyclopentadienyl transition metal compounds ("metallocenes") has been developed in striking contrast to the previous Ziegler-Natta catalysts which utilize aluminum alkyls as the activators. This new generation of catalysts demands more efficient methods for producing and utilizing alumoxanes. It is known that alumoxanes can be produced generally by contacting a trialkylaluminum with water under controlled reaction conditions to produce an alumoxane. However, in recent years this simple method has been expanded. WO92/21685, U.S. Pat. No. 4,908,463, U.S. Pat. No. 4,937,363, U.S. Pat. Nos. 4,968,827, 4,924,018, 5,003,095, 5,041,583, 5,066,631, 5,099,050, 5,157,137, 4,544,762, 5,084,585 and 5,064,797 all disclose various methods of producing alkylalumoxanes particularly methylalumoxane, to be used as a catalyst itself or as an activator for various catalysts such as mono, bis or tris cyclopentadienyl transition metal compounds. U.S. Pat. No. 4,952,540 discloses finely divided alumoxanes having an average particle size of 5 to 200 micrometers and a surface area of 20 to 1000 meters square per gram used in conjunction with a cyclopentadienyl transition metal compound to produce a polymer having high bulk specific gravity (also called bulk density). U.S. Pat. No. 5,015,749 discloses alumoxanes placed on a porous organic or inorganic aqueous imbiber material containing water. The support has an average surface area of 1 m.sup.2 /g to 1200 m.sup.2 /g and an average pore diameter of about 15 to about 10,000 angstroms.
While there are a multitude of references teaching various methods to produce alumoxanes it has not yet been discovered how to manipulate alumoxanes, specifically for maximizing supported catalytic activity and polymer product characteristics. Thus, a method is desired for manipulating alumoxane which produces catalysts with controlled bulk density, ash content, polymer particle sizes, and polymer particle size distribution.
During the polymerization process polyolefin catalyst systems comprised of co-catalyst/catalyst in a support become shattered into tiny fragments which end up being uniformly scattered throughout the final polymer product. The amounts of the individual residual elements relative to the total amount of polymer product are collectively referred to as the ash content. This is an important parameter from two points of view. First in some end product applications such as food packaging there are limits on the acceptable amount of ash, but even more important is the economic factor through the standpoint of catalyst efficiency. Lower ash contents are a direct effect of being able to make more polymer using less catalyst. Commercial polyolefin production reactors can be adversely influenced by inconsistencies in the individual behavior of any particular catalyst. A typical problem situation is one in which the catalytically active species become inhomogeneously distributed within the reactor to result in the build up of local "hot" spots. Here the temperatures get so high, that the product polymer melts and fuses together eventually forming internal chunks and causing reactor fouling. To ensure reactor operability it is preferred to run a catalyst system which maintains homogeneity of the active species throughout the reactor.
The ideal production scheme optimizes the final product properties as well as the physical form of the final product. Although the former of these can be optimized by means of chemistry and reactor conditions, the latter is controlled more by the fragmentation properties of the catalyst carrier which, in turn, are controlled by the dispersion of the catalyst on and throughout the carrier. Thus an important consideration in olefin polymerization has been developing methods for loading the carrier with catalyst. The location and chemical nature of the catalytically active species in the carrier microstructure is an important consideration. The exact arrangement of active catalyst sites within the carrier provides ultimate control over the carrier fragmentation behavior and, subsequently, over the physical form of the final product. Consequently, unambiguous knowledge of the supported catalyst microstructure can lead to control of final polymer product morphology, independent of the nature of the polymer being produced. This, in turn, provides control over the product bulk density as well as reactor operability and, as such, represents an improvement over the existing art.
In such reactor systems the amount of cocatalyst required to activate the catalyst is measured as the aluminum moles to metal moles ratio Al/M!. Depending on the particular process and catalyst there is a wide dispersion in values for this ratio, but often it is over 1000:1 and is typically over 500:1. Also large variations in bulk density values as well as intermittent reactor fouling are two problems often associated with alumoxane activated catalysts when they are supported on a silica carrier and used in a slurry or gas phase process.